Conventionally, a toner replenishing device that supplies toner from a toner bottle to a developing device is known, for example, as discussed in Japanese Patent Application Laid Open Nos. H10-198147 (JP-H10-198147-A) and 2009-80402 (JP-2009-80402-A). In a two-component developing system that utilizes two-component developer composed of toner and magnetic carrier, a toner density sensor is generally provided to detect density of toner included in the developer stored in the developing device by detecting the magnetic permeability of the developer. Then, replenishment of the toner is controlled based on a detection result to maintain a prescribed density thereof.
However, in an image forming apparatus employing the above-described toner replenishment control, actual toner density is sometimes different from that detected by the toner density sensor depending on dispersion of the toner stored in the toner bottle. In addition, an amount of toner to be supplied from the toner bottle sometimes fluctuates.
More specifically, when a prescribed amount of relatively hard toner (e.g. toner in an advanced state of agglomeration) is supplied to a developing device, a bulk of the developer decreases, and accordingly a bulk density (i.e., a density obtained by dividing a weight of developer in a container by a cubic capacity thereof when the developer as powder is stored in the container) increases more than when a prescribed amount of relatively soft toner (e.g. toner in a well-dispassion state) is supplied thereto. As a result, a distance between carriers in the developer decreases, and a permeability of the developer increases, so that an excessive amount of toner is supplied, that is, more than is ideal, than when relatively soft toner is supplied thereto.
Further, when toner in the toner bottle is relatively hard, and a mohno pump (i.e., a progressive cavity pump) used as a toner supply means is operated for a prescribed time period to supply such toner, a greater amount (i.e., weight) of toner is supplied and image density increases than when the above-described soft toner is supplied. As the image density increases, an excessive amount of toner is consumed and the number of images formed per unit amount of toner (i.e., yield) decreases. Further, various abnormalities, such as scattering of toner, background fogging, etc., occur due to excessive toner density.
To suppress such agglomeration of the toner, the toner replenishing device employed in each of JP-H10-198147-A and JP-2009-80402-A includes multiple toner bottles, and rotates one of them while replenishing the toner from another one of them.
However, in such a conventional toner replenishing device, although the toner in the toner bottle other than that replenishing the toner is soft (i.e., dispersed) due to the rotation of the toner bottle, the toner replenishing bottle immediately starts replenishing the toner without any preparatory rotation. Consequently, when toner is supplied after an extended period of inactivity, toner aggregates and is possibly supplied as is from the toner bottle to the developing device. In such a situation, it is possible to replenish the toner from the toner bottle to the developing device after rotating the toner bottle a prescribed times to soften the toner beforehand. However, when a toner bottle is always rotated by a prescribed number of times to soften the toner therein regardless of its aggregated condition, completion of toner replenishment to a developing device is delayed, and accordingly an apparatus downtime increases.